1. Field of the Invention
The present invention relates to an electrophotographic photoconductor comprising an electroconductive support and a photoconductive layer formed thereon, comprising an aromatic polycarbonate resin provided with mechanical strength, or both the mechanical strength and charge transporting properties according to the combination of structural units for use in the polycarbonate resin. In addition, the present invention also relates to aromatic polycarbonate resins with charge transporting properties, which are useful as the photoconductive materials for use in the electrophotographic photoconductor and as the materials for use in electronic devices such as organic electroluminescent (EL) device.
2. Discussion of Background
Recently organic photoconductors are used in many copying machines and printers. These organic photoconductors have a layered structure comprising a charge generation layer (CGL) and a charge transport layer (CTL) which are successively overlaid on an electroconductive support. The charge transport layer (CTL) comprises a binder resin and a low-molecular-weight charge transport material (CTM) dissolved therein. The addition of such a low-molecular-weight charge transport material (CTM) to the binder resin lowers the intrinsic mechanical strength of the binder resin, so that the CTL film becomes fragile. Such lowering of the mechanical strength of the CTL causes the wearing of the photoconductor or the formation of scratches and cracks in the surface of the photoconductor.
Although some vinyl polymers such as polyvinyl anthracene, polyvinyl pyrene and poly-N-vinylcarbazole have been studied as high-molecular-weight photoconductive materials for forming a charge transport complex for use in the conventional organic photoconductor, such polymers are not satisfactory from the viewpoint of photosensitivity.
In addition, high-molecular-weight materials having charge transporting properties have been also studied to eliminate the shortcomings of the above-mentioned layered photoconductors. For instance, there are proposed an acrylic resin having a triphenylamine structure as reported by M. Stolka et al., in "J. Polym. Sci., vol 21, 969 (1983)"; a vinyl polymer having a hydrazone structure as described in "Japan Hard Copy '89 p. 67"; and polycarbonate resins having a triarylamine structure as disclosed in U.S. Pat. Nos. 4,801,517, 4,806,443, 4,806,444, 4,937,165, 4,959,288, 5,030,532, 5,034,296, and 5,080,989, and Japanese Laid-Open Patent Applications Nos. 64-9964, 3-221522, 2-304456, 4-11627, 4-175337, 4-18371, 4-31404 and 4-133065. However, any materials have not yet been put to practical use.
According to the report of "Physical Review B46 6705 (1992)" by M. A. Abkowitz et al., it is confirmed that the drift mobility of a high-molecular weight charge transport material is lower than that of a low-molecular weight material by one figure. This report is based on the comparison between the photoconductor comprising a low-molecular weight tetraarylbenzidine derivative dispersed in the photoconductive layer and the one comprising a high-molecular polycarbonate having a tetraarylbenzidine structure in its molecule. The reason for this has not been classified, but it is suggested that the photoconductor employing the high-molecular weight charge transport material produces poor results in terms of the photosensitivity and the residual potential although the mechanical strength of the photoconductor is improved.
Conventionally known representative aromatic polycarbonate resins are obtained by allowing a 2,2-bis(4-hydroxyphenyl)propane (hereinafter referred to as bisphenol A) to react with a carbonate precursor material such as phosgene or diphenylcarbonate. Such polycarbonate resins made from bisphenol A are used in many fields because of their excellent characteristics, such as high transparency, high heat resistance, high dimensional accuracy, and high mechanical strength.
For example, this kind of polycarbonate resin is intensively studied as a binder resin for use in an organic photoconductor in the field of electrophotography. A variety of aromatic polycarbonate resins have been proposed as the binder resins for use in the charge transport layer of the layered photoconductor.
As previously mentioned, however, the mechanical strength of the aforementioned aromatic polycarbonate resin is decreased by the addition of the low-molecular-weight charge transport material in the charge transport layer of the layered electrophotographic photoconductor.
In recent years, aromatic polycarbonate resins with excellent sensitivity and electrical characteristics have been found as described in Japanese Laid-Open Patent Application 9-297419. However, at the present stage, those conventional aromatic polycarbonate resins are not always satisfactory in terms of the durability necessary for the electrophotographic photoconductor.